The purpose of a blockade is too make people's lives miserable enough until they succumb to your demmands. What you're proposing is just to box them up in their country and possibly bomb them? From the endurance the Japanese demonstrated during the war, that would not work, and even if it did, such a blockade would take a very long amount of time, and the constant bombings might even claim more lives. Again, the fire bombings are an example of this.

Considering that they were already weak and going to surrender, boxing up their country and taking out their military bases surely would be the final straw for them. No need to slaughter innocents.